The present invention relates to an RF coil for an MRI apparatus, a method of using the RF coil for an MRI apparatus, and the MRI apparatus. More particularly, the present invention relates to an RF coil for an MRI apparatus having a Figure-8 configuration for receiving a magnetic resonance signal from an object to be detected, a method of using the RF coil for an MRI apparatus, and an MRI apparatus using the method. The coil having a figure-8 configuration is referred to herein as an 8-shaped coil.
The MRI apparatus generates a magnetic resonance signal by the aid of a magnetic resonance phenomenon, and takes a tomographic image of the object to be detected. In the MRI apparatus, an improvement in the efficiency of the RF coil that transmits the RF pulse and receives the magnetic resonance signal is an important purpose that leads to an improvement in the image quality and a reduction in the imaging time.
FIGS. 7(a) and 7(b) are diagrams showing the 8-shaped coil. FIG. 7(a) is a diagram viewing the 8-shaped coil 2 from the above, and FIG. 7(b) is a diagram viewing the 8-shaped coil 2 along a direction indicated by an arrow of FIG. 7(a). The 8-shaped coil 2 forms two loops of a conductive path, and the conductive path crosses at an x-point and a y-point in the centers of those loops. The 8-shaped coil 2 is used to transmit an RF pulse and receive a magnetic resonance signal in the MRI apparatus (for example, refer to Patent document 1).
The conductive path of the 8-shaped coil 2 is ideally a→b→c→d→e→f→g→h→a. However, because the conductive path are overlapped with each other through a thin insulator at the x point and the y point in the center of the 8-shaped coil 2, a floating capacitance 3 that is determined according to a geometric configuration of an overlapped portion is developed between the x point and the y point. For that reason, a current if that flows through the floating capacitance 3 exists in the overlapped portion of the conductive path. When it is assumed that the frequency of the transmitting RF pulse or the frequency of the received magnetic resonance signal (hereinafter referred to as “the frequency of the magnetic resonance signal”) is ω, and the magnitude of the floating capacitance 3 is Cf, an impedance Z between the x point and the y point is represented by Expression 1. The frequency ω is also represented by f=ω/2π.
                              Z          =          -                ⁣                              j                          ω              ⁢                                                          ⁢                              C                f                                              =          -                ⁣                  j                      2            ⁢            π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          C              f                                                          [                  Ex          .                                          ⁢          1                ]            
From the above expression, when the floating capacitance 3 is large, or when the frequency ω is high, the impedance Z becomes small, and the current if increases.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-306442.
Up to now, because the frequency of the magnetic resonance signal of the MRI apparatus is low to the degree of about 64 MHz or lower, the impedance Z between the x point and the y point is large, the current if that flows through the floating capacitance 3 is slight. However, in recent years, in order to improve the image quality of the reconstruction image, there has been developed an MRI apparatus that exceeds 100 MHz in the frequency of the magnetic resonance signal. It is impossible to ignore the effects of magnetic coupling between the right and left loops of the 8-shaped coil 2 which is developed by the current if, or magnetic coupling between the right and left loops and another transmission RF coil or another receiving RF coil. For that reason, when the current if is large, it is necessary to add two or more decoupling circuits (at least one decoupling circuit in each of the right and left loops) for removing the magnetic coupling.
Also, even in the case where the frequency of the magnetic resonance signal in the MRI apparatus is low, when the 8-shaped coil 2 that is large in the configuration and large in the area of the overlapped portion of the conductive path is used, the floating capacitance 3 increases. For that reason, it is impossible to ignore the effect of the magnetic coupling between the right and left loops due to the current if, likewise.
It is possible to suppress the floating capacitance 3 to a small value by narrowing the width between the crossing points of the 8-shaped coil 2. However, in this method, because the resistance of the conductive path in the crossing points is large, the transmission efficiency of the RF pulse and the receiving sensitivity of the magnetic resonance signal in the 8-shaped coil 2 deteriorate.